(1) Field of the Invention
The invention relates to a lead lag damper for a main rotor unit of a rotary wing aircraft, e.g. a helicopter, with a rotor and a hub.
(2) Description of Related Art
Each degree of freedom of a rotor can be described by N rotor modes, with N being the number of blades of said rotor. There are cyclic modes (longitudinal and lateral), collective modes and differential modes.
The critical regressive rotor inplane mode is a non-harmonic cyclic mode. Rotary wing aircrafts main rotors feature a lead lag damping device acting on said inplane motion of the rotor blades by transforming shear deformation in the lead lag damping device into heat. Said lead lag damping is necessary for preventing aeromechanical instabilities of the helicopter, so called ground and air resonance caused by an interaction between modes of the fuselage and the regressive rotor inplane mode composed by the lead-lag oscillation of the individual rotor blades as a consequence of a circular rotating displacement of the centre of rotating rotor blades relative to the hub of a helicopter. The design and lay-out of a lead lag damping device depend on the applied rotor concept.
The document U.S. Pat. No. 5,372,478 describes a rotorcraft rotor with blades hinged in flap and lag. A common lag damper system is centered on an axis of rotation of the rotor. A squared shaped damper comprises planar strips with rigid plates superposed along the rotor axis, with elastomer layers. Strips and layers are resiliently interconnected. Each rigid plate of this central common lag damper system is connected only to one distinct blade root, via a dedicated ball joint.
The document U.S. Pat. No. 4,297,078 discloses a helicopter rotor with a rotor head, arranged to support a plurality of rotor blades for rotation about an axis. The rotor head comprises a generally hollow structure. Each rotor blade is supported by an elastomeric bearing arranged to permit blade flap, lead/lag and feathering movements. The support means comprises interconnected inner and outer elastomeric assemblies, the outer assembly being connected to each rotor blade through a common elastomeric lead/lag damper unit which provides inter-blade damping of blade lead/lag movements. The elastomeric assemblies are pre-compressed.
The document EP0424267 describes drag elastic-return damping devices for rotorcraft rotor blades. Such a device comprise a member for elastic return with incorporated damping, of the rotary type and whose functioning is ensured by circumferential shearing of a viscoelastic material highly stable to distortions, such as an elastomer. This ensures an elastic return of the blades to a neutral position in drag. A connecting rod drives, by way of a rotational joint, a lever and an outer armature of the corresponding member in rotation about an inner armature. This inner armature is retained in a fixed position relative to the hub. Flanges are fixed to the inner armature, onto a zone of the periphery of the hub, between two adjacent blades.
The document WO2010068194 describes a rotor system with a lead stop mounted to a spindle and a lead stop plate mounted to a rotor hub arm, this stop being operable to contact the lead stop plate. The rotor hub has a plurality of rigid hub arms.
The document US2007071602 discloses a rotorcraft rotor comprising a hub and at least two blades hinged by respective joints relative to the hub about respective pitch axes and about respective flap axes and about respective lag axes. Each blade is secured to a respective pitch lever. The rotor includes a respective pitch control rod connected to each pitch lever by a first ball joint and a common lag damper system centred substantially on the axis of rotation of the rotor. Connected to each of the blades; the damper system is secured to a respective lateral projection from each blade via a respective second ball joint that is sufficiently close to the first ball joint connecting the pitch rod to the pitch lever for the lag damper system to be relatively insensitive to flap oscillations of the hinged blades. The damper when seen from above is a square frame; each “side” of the frame forms a respective damper unit, these four identical damper units being connected together in pairs.
Said existing concepts for lead-lag dampers of the quoted state of the art have in common that each blade of the rotor has a hinge and an individual elastomeric damping device associated. The elastomeric material of said dampers undergoes a linear shear deformation due to the applied circular rotating displacement. The inplane motion of each of the blades of a rotor has to be taken into account said inplane motion comprising n/rev (n=0, 1, 2, 3 . . . −rev=revolution) and non-harmonic displacements. Said non-harmonic displacements are from the inplane eigenmodes of the blades of a rotor. Said inplane eigenmodes are superposed with said n/rev displacements. The coupling of said circular, non-harmonic rotating displacement with an airframe mode can yield the phenomenon of ground resonance, if the embedded damping is not sufficient.